The present invention relates to an oscillator circuit and, more particularly, to an oscillator circuit suitable to use in a picture-in-picture (PIP) system for displaying, on a common display screen, a first picture image (referred to as a "parent picture image" hereinafter) and a second picture image (referred to as a "child picture image" hereinafter).
A PIP system is for displaying, both a parent picture image and a child picture image on a common display screen and is commercially put into a practical use as a television set equipped with a PIP function. Since the child picture image is displayed on a limited portion of the display screen in lieu of the corresponding portion of the parent picture image, the child picture image to be displayed is derived by compressing an original child picture image.
In the case where the original child picture image, which is generated by the NTSC method having 262.5 horizontal scan lines per 1 field (1 screen), is compressed to, for example, one-third, it is necessary to derive image information of about 87 scan lines from image information of 262.5 lines. The child picture image thus compressed is further required to be displayed on a predetermined position on the display screen.
For these purposes, the original picture image is sampled in synchronism with a clock signal having a sampling rate corresponding to the required compressibility. The image information thus sampled is converted into digital data which are in turn temporarily stored in a memory. The reading-out operation digital data thus stored is initiated at a time point at which a scanning in a display device (CRT) proceeds to the position of the display screen on which the child screen image is to be displayed. The read-out data are then converted into an analog signal in synchronism with a clock signal having the same rate as the sampling rate. The compressed child picture image is thus produced and supplied to the display device.
When a picture image from a broadcasting station is selected as a child picture image, the image method thereof is the same as that of a parent picture image. That is, in a television set used in Japan or the United States, the parent and child picture images is generated by the NTSC method. In a television set used in European countries, on the other hand, they are of a PAL method.
A video signal from a VTR, that is, a picture image recorded on a video tape is sometimes used as the child picture image, however, in this case, the picture image may be recorded in NTSC or PAL method that is different from the parent picture image.
Since the PAL system having 312.5 horizontal scan lines per 1 field, an image obtained by compressing a PAL method image to one-third required image information of about 104 scan lines.
It is usual that the internal construction of the television set having the PIP system is primarily determined to accommodate to the NTSC method in Japan or the United States or the PAL method in Europe. That is, the sampling rate for compressing the original child picture image is determined and fixed in accordance with the image method of the television set. For this reason, when the child picture image of the PAL method is displayed on a television set of the NTSC system, the displayed child screen image is vertically reduced. This phenomenon will be described in detail with reference to FIGS. 7 and 8.
FIG. 7 illustrates an example in which the PIP system is incorporated in a television set of the NTSC method. A parent picture image 801 is displayed on a display screen 800 according to the NTSC method. A child picture image 802 is displayed on a predetermined position on the display screen 800. Assuming that the child picture image 802 is of the NTSC method, since both of the parent and child picture images 801 and 802 are of the same image method, a circle as the child picture image 802 is displayed as it is without a substantial deformation, as shown in FIG. 7(a). On the other hand, when the child picture image is of the PAL method, the compressed picture image actually has the image information of 104 scan lines in spite of a fact that the compressed picture image has only to have the image information of 87 scan lines. As a result, the circle as the child screen image 803 is displayed with a vertically reduced ellipse, as shown in FIG. 7(b).
On the other hand, in a television of the PAL system, a circle as the child picture image 812 of the PAL method is displayed as it is, as shown in FIG. 8(a). However, when the child picture image of the NTSC method is displayed, the compressed image thereof actually has image information of 104 scan lines while it is required to have images information of 87 scan lines. As a result, a circle as the compressed child picture image 813 is actually displayed with a vertically expanded ellipse, as shown in FIG. 8(b).
Thus, if the video method of the child picture image is different from that of the parent picture image, the child picture image is actually displayed in the screen with distortion.